1. Field of the Invention
The subject invention is directed to an information distribution system, particularly adapted for cable television (CATV) broadcasting.
2. Description of the Prior Art
During the past three decades, television has become standard in most American homes. For the most part, television systems have operated utilizing VHF and UHF signals transmitted by powerful broadcasting stations through the atmosphere and received by antennas connected to individual television sets. However, in recent years cable television system have become increasingly popular. In the typical cable TV system, the television signal is transmitted to individual subscribers through coaxial cable. Each subscriber's unit is tapped into a trunk line which carries the signal for the entire distribution network.
However, conventional coaxial cable systems are somewhat constrained by the number of channels which they can distribute. In order to penetrate urban areas with cable TV systems, operators have been required to provide many more channels than are offered in rural areas. To achieve this capacity, virtually all existing CATV systems employ a frequency division multiplexing transmission scheme. All available video channels are frequency division multiplexed and simultaneously transmitted on a coaxial cable to the subscriber. The subscriber unit is equipped with a converter which receives the multiplexed video channels, selects the desired channel and converts it to the appropriate viewing frequency, typically corresponding to channels 2, 3 or 4 on the subscriber's television set. Thus, the converter performs essentially two functions, namely channel selection and electronic frequency conversion. Each of these functions requires different component parts in the subscriber unit. The components required for the electronic frequency conversion function are relatively expensive and susceptible to tampering by subscribers. The problem of subscriber tampering is particularly acute in systems which offer different tiers of service at different prices. In such systems, subscribers may attempt to gain access to channels they have not paid for by tampering with the frequency converter in their unit.
While systems have been proposed to separate the frequency conversion function from the channel select function to eliminate susceptibility to theft of unauthorized signals, as in U.S. Pat. No. 4,064,460, they have not proved commercially attractive due to the fact that they require relatively complex and expensive electronic switching stations.
In view of the shortcomings of the conventional wired broadcasting system, particularly in urban areas, a "ramified" distribution network is proposed by Grodner et al in U.S. Pat. No. 3,751,670. In this system, the television signal is transmitted directly to an individual subscriber from a distribution center using optical signals as the transmission medium. The optical signals are transmitted in free space from the distribution center to a receiver at the subscriber station. Mirrors are used when it is necessary to change the direction of the signal to avoid obstacles. Each subscriber station is also equipped with means for generating and transmitting an optical signal to the distribution station to request service.
Since each individual subscriber receives a single signal directed from the distribution station, there is no need to employ frequency division multiplexing in providing multi-channel service to the subscriber. This permits more precision in measuring the use of the system by each subscriber for accounting purposes.
However, the ramified distribution network exhibits different, but equally serious, shortcomings. For example, because each subscriber station must be within signalling range of a distribution station, many distribution stations are required to service an area which would normally be served by a single broadcasting station. This is especially true in an urban area where buildings limit the number of straight, unobstructed paths which are required for transmitting optical signals between the distribution and subscriber stations. The only way in which the optical signals can be made to change direction is by interposing a series of mirrors, which presents other obvious disadvantages. Additionally, weather conditions can adversely affect the quality of the optical signals transmitted through free space, thereby causing poor reception at the subscriber station.
Fiber optics have been used in a limited number of cable TV trunk lines and, in at least one case, for distribution to homes in Japan. The Japanese system, known as Hi-OVIS, connects approximately 160 homes with a two-way switched video system. A highly complex and expensive video switch is required to direct 32 channels to 168 different outputs. Transmission is done at baseband on plastic clad silica fibers using LED sources. The system requires two optical fibers for each subscriber unit to provide two-way video service.
While the Hi-OVIS system avoids the problems associated with open air optical signalling, because of the complexity and expense of the video switch and the use of two-way video service, the system is far from economically viable.
Another system employing fiber optics is disclosed in U.S. Pat. No. 4,135,202. In this system, a common optical fiber connects a plurality of subscribers with a central broadcasting station. Hence, the system is similar to the trunk line system which conventionally employs coaxial cable. In an alternative embodiment, the patent discloses connecting a plurality of subscribers to the central broadcasting station with a plurality of individual optical fibers. Each subscriber is further connected to the central station by one or more individual auxiliary transmission lines.
Recently, a third TV distribution system utilizing optical fiber was suggested by K.Y. Chang (CLEOS meeting, San Diego, Calif. 2/27/80). This system contemplates using a single optical fiber for bidirectional signalling between a central switching node and the subscriber unit. At the central switching node, signals from a variety of sources are frequency division multiplexed and delivered to the subscriber through a single optical fiber. Here again, the switching mechanism at the central switching node and subscriber unit is quite complex since the various input signals must be multiplexed at the central switching node before transmitting them to the subscriber where they must be demultiplexed for channel selection.
While it is evident from the foregoing that numerous systems have been proposed for distributing information to a plurality of individual subscribers, there is, as yet, no system which is particularly suited for cable TV distribution in an urban setting where tiered service and security from tampering and signal theft are imperative.